The SF Tracts data are in geographic coordinates (what CRS?).
Use spTransform to create a new layer in the UTM10 NAD83 CRS.
sftracts_utm10 <- spTransform(sftracts, CRS("+init=epsg:26910"))
head(sftracts_utm10)
## class : SpatialPolygonsDataFrame
## features : 6
## extent : 549029.7, 552338.1, 4180865, 4184030 (xmin, xmax, ymin, ymax)
## coord. ref. : +init=epsg:26910 +proj=utm +zone=10 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0
## variables : 10
## names : STATEFP, COUNTYFP, TRACTCE, AFFGEOID, GEOID, NAME, LSAD, ALAND, AWATER, pop14
## min values : 06, 075, 010700, 1400000US06075010700, 06075010700, 107, CT, 92048, 0, 2592
## max values : 06, 075, 016300, 1400000US06075016300, 06075016300, 163, CT, 311339, 0, 5311